The present invention relates generally to the field of sensing the concentration of an aqueous solution. More particularly, the present invention relates to ultrasound sensing of the concentration of methanol""s aqueous solution in connection with a fuel circulation loop of a direct methanol fuel cell.
A direct methanol fuel cell (DMFC) is a type of polymer electrolyte membrane fuel cell (PEMFC). A DMFC operates at low internal temperature (xcx9c80 C.) and fuel is a non-flammable dilute methanol/water mixture, closely approximating auto windshield wiper fluid. There are no Environmental Protection Agency (EPA) regulated emissions in connection with using a DMFC, allowing for continuous operation. By some estimates, projected engineering power density for DMFCs is 10-20 Watts per liter.
PEMFCs, such as DMFCs, provide low temperature operation, cleanliness, safety, quiet performance, ease of operation, low maintenance, portability, modularity, scalability, responsiveness and versatility. The xe2x80x98directxe2x80x99 aspect of DMFCs implies the elimination of a fuel reformer component, which simplifies the overall fuel system, lowering cost, making for a smaller assembly and further increasing efficiency. Due to the versatility of DMFCs, they may be used, e.g., to power transceiver stations for remote mobile telecommunications systems, to provide backup power to telecommunications systems and to provide remote residential power, among a myriad of other applications. Thus, DMFCs are capable of providing reliable and affordable power in remote locations. From an efficiency standpoint, current DMFC performance yields approximately 1 kilowatt average load for 3 months using about 500 gallons of fuel. Moreover, significant advances are expected in the near future increasing efficiency even further, bringing DMFCs ever closer to the forefront of power technology.
During operation of a fuel circulation loop for a DMFC, methanol and water are mixed in a fuel mixer, and it is important to the process that the concentration of methanol aqueous solution remains reasonably controlled throughout operation of the DMFC. In this regard, the rate by which methanol is added to the system is related to the rate of depletion of methanol in the system and thus, sensing the concentration of methanol so that an appropriate amount of methanol can be metered is desirable for such a process. Accordingly, there is a need for accurate measurement of fuel concentration in such a fuel cell system.
U.S. Pat. No. 6,306,285, to Narayanan et al., entitled xe2x80x9cTechniques for Sensing Methanol Concentration in Aqueous Environmentsxe2x80x9d (the ""285 patent) discloses a technique for sensing methanol concentration, and provides a methanol concentration sensor device for coupling to a fuel metering control system for use in a liquid direct-feed fuel cell.
The method of the ""285 patent teaches detecting a methanol compound concentration in an aqueous environment by using a sensor element to probe a liquid analyte solution including methanol to produce a sensor response. The sensor element includes an anode, a solid electrolyte membrane and a cathode. The sensor element also includes a catalyst, which is capable of chemically reacting with methanol. According to the method, the anode and cathode of the sensor element are immersed in the liquid analyte solution. An electrical power supply""s positive terminal is connected to the anode and the electrical power supply""s negative terminal is connected to the cathode. An analyte concentration sensing device is connected to the sensor element for detecting the response to the analyte, and is also connected electrically to the sensor element and the power supply in order to detect an amount of current consumed thereby. The method is alleged to be reliable in aqueous environments in the analyte concentration range 0.01 M to 5 M, and a temperature range of 0xc2x0-100xc2x0 Celsius.
The sensors of the ""285 patent, however, are relatively slow due to the time for the reactions to take place in the analyte solution. The need for a fast view of macroscopic change in concentration of methanol solution is thus undermined by the method taught by the ""285 patent. Additionally, the sensor of the ""285 is sensitive to both metallic and biological contaminants, skewing results undesirably. Since these contaminants further affect the operation of the DMFC itself due to a similar anode/cathode mechanism, over time, the use of the sensor of the ""285 fails to serve as an impartial diagnostic tool. Moreover, additional plumbing and components are required that make such an implementation complex. For example, separate plumbing must be provided to the fuel samples and separate oxygen feed.
There are also other exhaustive techniques that are capable of measuring the concentration of methanol at a very high resolution. Since these techniques reproduce one or another principle of measurement of absolute physical value connected to absolute value of concentration, they are expensive and cumbersome, and thus not affordable and simple.
Thus, there is currently a strong need in the art for an improved system and methods for measuring the methanol-water mixture concentration accurately for a stand-alone DMFC. In this regard, there is a need in the art for simple, affordable and reliable methods for measuring methanol""s concentration in aqueous solution in a DMFC fuel circulation loop.
In view of the foregoing, the present invention provides systems and methods for ultrasound sensing a compound""s concentration in aqueous solution. As a result of the ultrasound sensing, an accurate real-time measurement of the concentration of the compound of interest in aqueous solution is obtained. In various non-limiting embodiments, the invention provides ultrasound sensing of methanol""s concentration in aqueous solution in connection with a fuel circulation process for a direct methanol fuel cell. Since the speed of sound in a water-methanol system increases significantly with methanol content, techniques for measuring characteristic sound velocities are used to give sufficient resolution for a fuel circulation process.
Other aspects of the present invention are described below.